Access $_POST data after redirect

Redirects are GET requests usually, and the browser doesn’t send the POST data for those. That’s not something WordPress can change.

You could create a session, or – better – process the POST data first, then redirect. In your plugin, you could do:

add_action( 'plugins_loaded', 'process_post_data', 0 );

function process_post_data()
{
    // Read raw POST data, not touched by WordPress
    $data = file_get_contents( 'php://input' );

    // then redirect
}